Various projectors that are adapted to guide via a lighting optical system to a display device, such as liquid crystal panels, light emitted from a light source and project said light onto a screen via a lens device after modulating the light by the display device are known conventionally (e.g. Japanese Unexamined Patent Application Publication No. Hei-8-43729). In order to improve the contrast of a projected image, the lens device of a conventional projector may be provided with a diaphragm mechanism which has a circular aperture and is positioned within a group of lenses.
In the case of projectors that use liquid crystal panels or the like as the display device, it is necessary to ensure telecentricity between the display device and the lens device. Therefore, in order to improve the contrast of a projected image while limiting the reduction of telecentric properties, the entire lens device has to be designed to permit adjusting of the aperture diameters of the lens frames that secure the respective lenses.